For an in vivo assessment of skeletal muscle development in myostatin deficient mice, we have determined the skeletal muscle precursor frequency and muscle fiber number in normal and myostatin knockout mice. We are also analyzing the function of myostatin in muscle stem cells and fibers in vitro and in vivo. We are making genetically modified mice to manipulate myostatin signaling in specific cell progenitors using the inducible tetracycline system. Several transgenic mouse lines have been generated and are being analyzed for skeletal muscle phenotypes. We have analyzed the activation of satellite cells and their contribution to skeletal muscle hypertrophy after myostatin inhibition in adult mice.